1. Field of the Invention
The invention relates to a hybrid electric vehicle powertrain with separate power flow paths from an engine and from an electric motor to vehicle traction wheels through a multiple-ratio transmission.
2. Background Discussion
In a hybrid electric vehicle powertrain, there are two sources of power. One power source is an engine, which could be a spark ignition internal combustion gasoline engine, a diesel compression ignition engine, a gas turbine engine, a hydrogen fuel cell engine, etc. A second power source comprises at least one electric motor that is electrically coupled to a traction battery and to an electrical generator. Depending upon the operating mode of the powertrain, the generator may act as a motor and the motor may act as a generator. For example, the motor may provide charging voltage to the battery during an engine braking mode.
The two power sources can be configured either as a series hybrid powertrain or as a so-called parallel hybrid powertrain.
In a series powertrain configuration, the generator is powered by the engine. The generator, in turn, charges the battery, which powers the motor, which is coupled to traction wheels. There is no direct mechanical torque flow path from the engine to the wheels in such a series powertrain configuration.
In a so-called parallel hybrid electric vehicle powertrain, engine power and power from the battery and the electric motor are used separately or are combined to provide traction wheel power.
In a parallel/series hybrid vehicle powertrain configuration, the characteristics of a parallel configuration and a series configuration may both be available. A portion of the engine torque in such a parallel/series hybrid configuration is delivered through a mechanical torque flow path to the traction wheels, and a portion of the engine power drives the generator, which powers the battery, so that motor power can be delivered to the traction wheels. Kinetic energy of the vehicle can be captured during engine braking, thereby storing regenerative energy in the battery.
In a parallel hybrid electric vehicle powertrain configuration, the engine can be turned off during periods of low power demand as the motor provides the sole power source for the vehicle. The engine may be disconnected from the motor, when the engine is not running, by opening a disconnect clutch. When power demand increases, the engine can be restarted and the clutch can be re-engaged.
An example of a parallel hybrid electric vehicle powertrain may be seen by referring to U.S. Pat. No. 6,735,502, which is assigned to the assignee of the present invention. An example of a parallel/series hybrid electric vehicle powertrain may be seen by referring to U.S. Pat. No. 7,013,213, which also is assigned to the assignee of the present invention.
In a parallel hybrid electric vehicle powertrain, both the engine and the motor can be directly coupled to the traction wheels through a geared transmission so that both power sources can power the vehicle. The transmission typically has multiple gear ratios. An example of a transmission of this type is an electronically controlled converterless transmission disclosed in U.S. Pat. No. 6,299,565, which also is assigned to the assignee of the present invention.
A vehicle system controller is used to implement several functions in a hybrid electric vehicle powertrain with a parallel configuration. The three most fundamental functions are (1) transfer of motor power and engine power to the wheels based on driver demand for power; (2) efficient management of the energy in the battery and engine power; and (3) system fault management.
The vehicle system controller consists of two primary components. First, it has a logical state machine that describes both the operating modes of the vehicle as well as a set of rules for transitioning between the modes. Secondly, the vehicle system controller has an output command control for vehicle system components, including the engine and the transmission for each state within the state machine.
If a transmission ratio shift occurs in a parallel hybrid electric vehicle powertrain following a driver demand for torque when the powertrain is in an electric-only drive state, an engine start event can cause undesirable momentary reduced drivability performance because of engine inertia effects and a time lag between a command for engine power and a command response.